Modulation



Patented Apr. 28, 1942 MODULATION Lothar Bl'iick, Berlin,

7, assignor to Tele- German runken Gesellschaft fiir DrahtloseTelegraphic, Berlin, Zehlendorf, Germany, a corporation of GermanyApplication 3 Claims.

This application concerns a modulation circuit organization forradio-frequency transmitters comprising a rectifier. The presentinvention is an improvement of the system disclosed in Brucksapplication #336,546 filed May 22, 1940 now United States Patent No.2,269,140, dated January 6, 1942.

A modulation circuit arrangement for radiotrequency transmittershas beendisclosed in the said Bruck application which follows a straightline lawup to very high modulation percentages. The disclosure of the saidapplication is predicated upon a modulation scheme known'in the priorart in which a rectifier, for example, a diode, is connected'in serieswith a load resistance and the series combination connected to anoscillatory circuit included in the plate circuit of the characteristicof-the rectifier, direct current as a function of the direct-currentpotential for various radio-frequency potentials fed to the precedingtube, is covered by the modulating audio frequency as symmetrically aspossible. (3) The modulated radio-frequency potential is taken 03 fromthe oscillation circuit which is included in the plate circuit of thetube having a high internal resistance.

Now, the present invention is concerned with the problem to stabilizethe modulation percentage, in a circuit arrangement of the kinddisclosed in the said Bruck application, in spite of variations of theoperating potentials, that is to say, the direct-current voltages. Itwill be remembered that such fluctuations of the working voltage, in thefirst place. cause a change in the radio. frequency amplitude, and, inthe presence of unvaried modulation potential it change in the percentage of modulation, and this is undesirable,

March 4, 1941, Serial No. 381,624 GermanyNovember 8, 1939 quency in asecond rectifier and second load resistance.

Figs. 1 and 2 illustrate exemplified embodiments of my improvedmodulation system and Fig. 3- illustrates by graphs practical results ofthe invention.

Referring to Fig. 1, the amplifier tube V, the

" same as in the parent application, is fed with I or rated workingvoltages the direct-current volt- 0 ages across resistance RI and R2 areof like size,

the unmodulated radio-frequency potential Ulfl. The carrier is amplifiedin tube V and supplied to the secondary winding S oi. a transformer Tand thence to output means. The diode i has two electrodes 6 and itconnected in shunt to the tuned circuit Sch. Rectification in the diodei sets up current in the load resistance Bi. Now, this load resistanceRI, through the transformer Ti, is fed with the modulation frequency UNFfrom a source II. The modulated radio-frequency potential is as statedabove taken from the oscillation circuit Sch.

To stabilize the modulation percentage irrespective and independently offluctuations in the working voltage, according to an object of theinvention, a second rectifier 2 having electrodes I4 and II connectedwith loadresistance R2 is provided. This rectifier 2 is led with theaudio frequency by way of the transformer T2. The

condenser 02 serves for audio-frequency bridaing or by-passing oi theresistance R2, while the condenser Cl serves for the radio-frequencybypassing of the resistance RI. The direct-current voltage which is setup across the resistance R2 as a result of the rectification oi theaudio frequency is applied by way of the wire I8 to the left hand end ofthe resistance RI. The circuit is so balanced that in the presence ofnormal with the result that no direct current flows through the wire l8.

But, ii, for some reason or other; the audiofrequency potential or theradio-frequency potential changes, then a direct current willflowthrough the wire it. According to its direction this direct current willeither act in a damping or a boosting manner upon the radio-frequencycircuit. As a consequence, the radio-frequency amplitude is alwaysmaintained at a certain ratio to the audio-frequency amplitude so thatthe modulation percentage is approximately stabi- Suppose theaudio-frequency potential is constant, while the radio-frequency ptential ditained by rectification of the modulation frell minishes.

As a result, the direct-current potenaasnoso tial across RI below thevalue of that acting across resistance R2. The further result is that acurrent will be caused to flow through wire it tending to equalize thetwo potentials. The said direct current acts the very same way as if theleft hand diode were impressed with a negative biasing potential. As, aresult, the damping. efiect produced by the diode upon theradio-frequency circuit is lessened and the radiofrequency potential,acting at the circuit, rises so that the original modulation percentageis restored. Y 1

Fig. 2 illustrates an exemplified embodiment in which the need ofaudio-frequency transformers is eliminated. The input resistanceconsists here of the resistance R3. From a tap on the resistance R3 bylead 20 is supplied to the diode I at the same time the audio frequencyas well as the direct current voltage resulting from the rectificationof the audio frequency to the resistance RI By shifting the tap orslider along-R3 it is possi-- ble to adjust working conditions to anydesired modulation percentage. The load resistance R: is high comparedwith the resistance R3 to the end that, when making adjustments, thedirectcurrent voltage may change as little as feasible,

and that the modulating audio frequency may V not be distorted.

Fig. 3 shows in what 'way the modulation percentage m is changed upon avariation of the plate direct current voltage Ua of tube V, withoutapplication of the idea underlying the invention (Graph'l) and withapplication thereof (Graph 2). The curvature of the graphs is due tolimited measuring accuracy. It will be noticed that the amplitude ofsaid voltage, a source of modulating potentials, a second rectifiercoupled to said source of modulating potentials to rectify saidmodulating potentials to derive a second current the intensity of whichcorresponds to the amplitude of aid-mama; potentials, means for opposingsaid currents. to derive resultant current when the said two currentsare unequal, the direction of fiow of which depends on the relativeintensities of said first two currents, means-for impressing saidresultant current on said first rectifier to control the bias thereof,and a connection between said source of modulating potentials and saidfirst rectifier for controlling the instantaneous current therethroughin accord-- ance with the modulating potentials.

2. In a modulation system, a parallel tuned circuit wherein highfrequency wave energy to be modulated is caused to ilow,-a'firstrectifier and a first resistance in series connected in shunt ing saidsecond resistance in parallel with said I current is greater, saidpotential being applied to said first rectifier as a biasing potential,and means for impressing modulating potentials on one of saidresistances.

3. In a modulation system, a. parallel circuit wherein wave energy ofcarrier wave frequency is caused to flow, a rectifier and a loadresistance for said rectifier in series connected in shunt to saidparallel tuned circuit, a source of modulating potentials, means forimpressing said modulating potentials on said load resistance, and meansfor rectifying said modulating potentials to derive direct currentvoltage which corresponds to the envelope of the modulating potentialsand impressing the same on said resistance to thereby stabilize thepercentage modulation in the presence of. fluctuations in the waveenergy of carrier wave frequency.

.LOTHAR BRiicK.

tuned

